1. Field of the Invention
The present invention relates to a boundary microphone, and in particular to the boundary microphone characterized by its shielding structure.
2. Related Background of the Invention
A flat microphone to be installed on a desk or the like for use is an on-surface sound pickup microphone, and is called a boundary microphone or a surface mount microphone. In the present invention, a microphone of such a type is referred to as a “boundary microphone”. Since the boundary microphone is intended to be installed in a low profile manner on a desk or the like, the boundary microphone is designed so as to be flat with restricted height. Usually, the boundary microphone has a structure in which a metallic part having a plurality of openings for introducing sound waves is connected, one above the other, with a metallic part for supporting a microphone unit and an electronic circuit board. The microphone unit and the electronic circuit board are disposed in a space formed by the upper and lower metallic parts.
The boundary microphone is constructed of mainly two parts, i.e., a base and a cover in appearance. Electric circuits, such as the microphone unit, the circuit board, an impedance converter, a tone control circuit, and an output circuit are incorporated into the internal space formed by the base and the cover. The base is made of a flatmetal and can be secured to a desktop or a floor surface. The cover is made of a metallic part, and has numerous openings formed therein in order to introduce sound waves to the microphone unit. For the cover, a punching metal having numerous holes formed therein by punching is often used, for example. The cover is put on the base and secured to the base with a screw.
If high frequency noise caused by electromagnetic waves enters into the above-described electric circuit from the outside, this noise is detected by a semiconductor element used in the impedance converter or the like, and is then mixed into a sound signal as a noise signal, thereby degrading the signal to noise ratio (S/N) at the microphone output. Accordingly, the connecting portions in the peripheral portions of the base 1 and the cover 2 are preferably joined together without any gap so as to shield the internal components from high frequency noise coming from the outside. The reason is, if the connecting portions are not joined together appropriately but the base and the cover are in point contact with each other and there is a gap therebetween, high frequency noise will enter from this gap.
However, the conventional boundary microphone has such a structure that the base and the cover tend to be in point contact with each other for the reason described below. Although the base is typically manufactured using a zinc die casting method or the like, the casting surface is not flat. On the other hand, for the cover, a punching metal is typically used as the material as described above, and by press molding this, the cover is formed into a desired shape. However, the peripheral portion, which is electrically and mechanically in contact with the base, is not flat and there occur irregularities. Accordingly, even if the cover is directly put on the base and is screwed thereinto, the base and the cover will be in point contact with each other.
For the electromagnetic waves of VHF and UHF bands which are used in the conventional TV broadcast and the like, the shielding effect can be exhibited even if the base and the cover are slightly in point contact with each other. However, as in recent years, when mobile phones using radio waves in a shorter wavelength region become popular, the electromagnetic waves will enter into the internal space of a microphone even when there is a very small gap, because they have a short wavelength. In addition, the mobile phone is an item which a user uses nearby, and therefore the mobile phone will be more often used near a microphone, thus creating an environment in which electromagnetic wave noise is more likely to enter into the microphone. This is true of the boundary microphone that is used in a conference room and the like.
In order to exhibit a shielding effect even if a mobile phone is used near a boundary microphone under such environmental condition, the present inventors have previously proposed a boundary microphone provided with a microphone case capable of exhibiting a sufficient shield ability against strong electromagnetic waves of a portable telephone level (see Patent Document 1). This technique is characterized in that a gasket having both elasticity and electric conductivity is disposed between a base and the peripheral end face of a cover constituting the housing of the boundary microphone.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2005-333180